This invention relates to the retorting of hydrocarbon-containing solids, particularly shale.
Shale oil is not a naturally occurring product, but is formed by the pyrolysis or distillation of organic matter, commonly called kerogen, formed in certain shale-like rock. The organic material has limited solubility in ordinary solvents and therefore cannot be recovered by extraction. Upon strong heating, the organic material decomposes into a gas and liquid. Residual carbonaceous material typically remains on the retorted shale.
In its basic aspects, the retorting of shale and other similar hydrocarbon-containing solids is a simple operation. The major step involves the heating of the solid material to the proper temperature and the recovery of the vapor evolved. However, for a commercially feasible process, it is necessary to consider and properly choose one of the many possible methods of physically moving the solids through a vessel in which the retorting is to be carried out as well as the many other variances in operating parameters, all of which are interrelated. The choice of a particular method of moving the solids through the vessel must include a consideration of mechanical aspects as well as the chemistry and the processes involved. Further, it is necessary to consider the many possible sources of heat that may be used for the pyrolysis or destructive distillation.
In order to achieve a retorting process that is economically attractive and one which produces the maximum amount of high-quality shale oil, the various operating parameters must be controlled so that the overall process is economical, continuous and highly reliable. Any equipment usable in the process must permit a high throughput of material since enormous quantities of shale must be processed for a relatively small recovery of shale oil. Process equipment for shale must have a high thermal efficiency and as in the case of all mechanical devices, the retorting equipment should be as simple as possible so that a relatively proven and economically attractive mechanical device may be utilized in the operation of the retort.
In an effort to provide an economically commercial process, literally hundreds of retorting processes have been proposed, each of which offer a somewhat different choice and/or combination of the many possible operating conditions and apparatus.
One problem with many prior art processes is that the quality of shale oil obtained is relatively low. A common problem with many prior art processes is the long residence time at high temperatures which results in many secondary and undesirable side reactions such as cracking, which may increase the production of normally gaseous products and decrease the yield and quality of the condensable product. Another problem with many prior art processes is that a portion of the shale oil is combusted, which also leads to a decrease in the yield of condensable hydrocarbons. Thus, in any process designed to produce the maximum yield of high quality condensable hydrocarbons, it is preferred that the retorting take place in the absence of molecular oxygen and that the volatilized hydrocarbons are quickly removed from the retorting vessel in order to minimize deleterious side reactions such as cracking or polymerization.
The quality and yield of shale oil produced is greatly dependent upon how the retorting process is operated. For example, the raw shale can be heated rapidly or slowly and the shale can be finely divided or vary widely in size. These and many other factors greatly influence the quality and quantity of the shale oil produced and the overall thermal efficiency of the process. In essentially all processes for the retorting of shale, the shale is first crushed to reduce the size and time necessary for the retorting process. During the crushing or mining of the shale, it is difficult to obtain uniformly sized pieces and/or costly to separate the crushed shale into various sizes. Also, it is extremely expensive to crush all of the shale to a very small uniform size. It is therefore desirable to have a retorting process which can accommodate a wide-size range of solids.
Another problem with many prior art processes, particularly with countercurrent flow processes, is that after the shale oil has been vaporized, it then comes in contact with countercurrent flowing solids which are at a much cooler temperature, which leads to condensation of a portion of the shale oil and reabsorption of a portion of the vaporized shale oil into the down-flowing shale. This condensation and reabsorption leads to coking, cracking and polymerization reactions, all of which are detrimental in regard to producing the maximum yield of condensable hydrocarbons.
In one aspect of the present invention, a counter-current flowing stripping gas is utilized. Retorting processes utilizing countercurrent flow of a stripping gas are well known in the art. For example, U.S. Pat. No. 3,736,247 describes a process wherein shale is fed into the top of a vertical retort and moves downward countercurrent to the flow of an upward flowing stripping gas. However, the upward-moving stripping gas contains oxygen and a portion of the shale oil is combusted in the retort, leading to decreased yields of normally liquid hydrocarbons.
Retorting processes using sand or other solid heat-transfer materials are also known in the art as shown, for example, in U.S. Pat. No. 2,788,314.
A gas lift retorting process is described in U.S. Pat. No. 3,501,394. However, in this patent, shale fines are completely retorted by contact with a noncombustable lift gas. Relatively long contact times of 5 to 30 seconds are required to completely retort the shale which requires a very long lift tube. Furthermore, only shale fines can be used in the process and it is very expensive to crush shale down to the size of fines.
Disclosed in U.S. Pat. No. 3,925,190 is a retorting process wherein the shale is preheated in a series of risers. The temperatures are carefully controlled in the risers to avoid premature pyrolysis. Furthermore, the shale is preheated by combusting a portion of the shale with an oxygen-containing gas.
Lift-type combustors involving the combustion of a hydrocarbon-containing solid are also well known in the art as shown, for example, in U.S. Pat. No. 3,501,394.
The present invention overcomes these and other problems and provides a new retorting process that achieves high oil recovery at high retorting rates, permits accurate control of retorting temperatures, avoids combustion of products with oxygen, minimizes degradation of the condensable product, minimizes the gaseous products and simultaneously processes a wide-size range of shale at high retorting efficiency.